Electrochemical devices, such as metal-air batteries or metal-air fuel cells, are very promising energy conversion technologies that provide alternatives to the use of fossil fuels. As is known in the art, typical metal-air batteries and fuel cells comprise anodes that are formed using metals such as zinc (Zn), aluminum (Al) and lithium (Li). During the discharge of such batteries and fuel cells, oxidation of the metal occurs at the anode, which releases electrons which are transported via an external circuit to a cathode. At the cathode, an oxygen reduction reaction occurs, converting oxygen from air and water from an electrolyte into hydroxide ions. In zinc-air batteries in particular, hydroxide ions then migrate through the electrolyte to reach the anode where they form a metal salt (e.g. zincate), which decays into a metal oxide (e.g. zinc oxide). As such, the metallic anode gradually becomes depleted over time in a primary metal-air battery or fuel cell, thus requiring a continuous supply of metal for long term operation. However, the depletion of the anode can be mitigated by introducing oxygen evolution reaction at the cathode while the battery or the fuel cell is not being discharged, as this result in the oxygen reduction reaction to occur at the anode, which in turn causes metal to be regenerated at the anode. However, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) have large overpotentials and sluggish reaction kinetics. Therefore, to realize large scale application of metal air battery/fuel cells, improved catalysts are required.
Various approaches have been proposed to address the abovementioned deficiencies, such as through the use of bifunctional catalysts. Bifunctional catalysts are generally catalysts that catalyze both oxygen reduction and oxygen evolution reactions. For example, Jörissen (Ludwig Jörissen (2006); “Bifunctional oxygen/air electrodes”; Journal of Power Sources 155 (1): 23-32) reviewed many bifunctional catalysts, which catalyze both ORR and OER, made with various materials such as perovskite, spinel and pyrochlore type mixed metal oxides. However, Jörissen indicates that additional research is still needed in the field of bifunctional catalysts. In another example, Lu et al. (Yi-Chun Lu et al. (2010); “Platinum-Gold Nanoparticles: A Highly Active Bifunctional Electrocatalyst for Rechargeable Lithium-Air Batteries”; Journal of American Chemical Society, 132 (35): 12170-12171) describe a bifunctional catalyst based on platinum and gold; however, the high cost of the catalyst discourages its large scale implementation.
U.S. Publication No. 2007/0166602 to Burchardt describes combining an oxygen reduction catalyst and various oxides (e.g. CoWO4, La2O3) to obtain high bifunctional activity. U.S. Publication No. 2007/0111095 to Padhi et al. describes using manganese oxide contained in an octahedral molecular sieve as a catalyst for metal-air cathodes.
Other approaches are also known which generally involve selecting one catalyst for catalyzing the oxygen reduction reaction and another for catalyzing the oxygen evolution reaction and combining the two catalysts together to effectively obtain a catalytic material that catalyzes both reactions. However, these approaches add complications to electrode fabrication and increase the cost of production. Furthermore, in most of these approaches, the catalyst is either an expensive precious metal like platinum (Pt) or gold (Au), or a mixture, composite or spinel of oxides containing other expensive materials such as lanthanum oxide (La2O3).
A further example of a catalyst is provided in Applicant's co-pending PCT application number PCT/CA2012/050050, filed Jan. 27, 2012, the entire contents of which are incorporated herein by reference.
Graphene is a material that is known to have unique properties such as high chemical resistance and high electrical conductivity, among others. More importantly, graphene is generally prepared from graphite, which is inexpensive and abundant. It is well known in the art to dope graphene with other elements to increase its activity for use as a catalyst. Several researches have explored the effects of doping graphene with elements such as nitrogen and boron for various applications. Although the effects of doping graphene with sulfur has been studied by Yang et al., (Zhi Yang et al. (2012); “Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction”; ACS Nano 6 (1): 205-211) only its effects on oxygen reduction reactions (ORRs) were studied. Furthermore, the sample preparation method used by Yang et al. involved reacting aromatic compounds containing sulfur at high temperatures, thus giving rise to a relatively high cost of production as well as potential health and/or environment issues.
The prior art also describes the beneficial effects of mixing carbon materials with sulfur for electrochemical energy applications. For example, U.S. Publication Number 2009/0311604 to Stamm et al. describes mixing carbon material with sulfur to prepare the electrodes for a lithium-sulfur battery. In another example, in U.S. Publication Number 2012/0088154 to Liu et al., graphene-sulfur nanocomposites were used as an electrode material for a rechargeable lithium sulfur battery.
Despite the various proposed approaches as discussed above, there remains a need for a catalyst material that addresses at least one of the deficiencies known in the art. For example, there exists a need for cost effective catalysts that: (i) possess improved activity, (ii) possess improved stability, and/or (iii) do not contain expensive materials such as precious metals. Furthermore, there exists a need for a method of producing catalysts in a cost effective manner and which does not harm the environment.